1. Field of the Invention
The present invention relates to an ionization type smoke sensing device. More particularly, the invention relates to an ionization type smoke sensing device which includes an ionization chamber using a radioactive source and operates to detect smoke flowing into the ionization chamber.
2. Description of the Prior Art
There has been put into practical use an ionization type smoking sensing device including an ionization chamber using a radioactive source and adapted to detect smoke flowing into the ionization chamber on the basis of the amount of an electric current flowing through the ionization chamber. FIG. 1 is a schematic view of an example of a conventional ionization type smoke sensing device. The smoke sensing device shown in FIG. 1 includes an anode electrode 2 and a cathode electrode 4, which define an ionization chamber 7. The anode electrode 2 is in the form of a bottomed cylinder whose peripheral surface is formed with smoke inlet ports 1. The anode electrode 2 and cathode electrode 4 are both made of heat-resistant metal, such as stainless steel. The cathode electrode 4 is dish-shaped, having a radioactive source 3 attached to the inner surface thereof. The cathode electrode 4 is fixed to the anode electrode 2 by screws 6 with an insulating material 5, such as a ceramic material, interposed therebetween. Therefore, the ionization chamber is defined by the anode electrode 2 and cathode electrode 4. Current detecting means (not shown) is provided for detecting an electric current flowing between the anode electrode 2 and cathode 4. If smoke, e.g., from a fire flows into the ionization chamber 7 through the smoke inlet ports 1, the amount of electric current flowing between the anode electrode 2 and cathode electrode 4 changes, whereby the presence of smoke is detected.
Such ionization type smoke sensing device using a radioactive source 3 must be so arranged that even if it is subjected to intense heat from, e.g., a fire, the radioactive source 3 will not be scattered out of the ionization chamber 7 and lost. To this end, as shown in FIG. 1, the conventional ionization type smoke sensing device uses the insulating material 5 through which the anode electrode 2 and cathode electrode 4 are fixed together, and a heat-resistant material, such as a ceramic material, is used as such insulating material. However, ceramic materials are not only expensive but also are hard to process, thus making smoke sensing devices themselves expensive.